In a conventional CVD reactor for depositing semiconductor material on a monocrystalline substrate of semiconductor material, a decomposable gas such as silane or trichlorosilane or dichlorosilane is directed over the surface of a silicon wafer to decompose on contact with the hot wafer and deposit silicon on the silicon wafer. Such decomposition commonly occurs in a cold-wall reactor to reduce the decomposition of the gas and the deposition of one or more constituent elements of the gas on the walls of the reactor. Such depositions on the walls of the reactor may accumulate and constitute a source of contamination in subsequent epitaxial depositions on a series of wafers being processed, and may also obliterate the transmission of high-intensity light flux through a window-wall of the reactor from an external light source to the wafer heated thereby within the reactor.
In addition, since gaseous decomposition and deposition of the CVD layer depend critically upon reaction temperature, various schemes are commonly employed to assure substantially isothermal temperature distribution over the entire area of the wafer. In some known reactors, a wafer is supported by a susceptor that is disposed in thermally conductive contact with the underside of the wafer to assure substantially uniform temperature distribution over the area of the wafer. However, a susceptor positioned on the underside of a wafer substantially to serve as an isothermal mass across the dimensions of the wafer also significantly increases the time required to elevate the temperature of the wafer (typically to temperatures of about 1000.degree. C.), and also tends to contaminate the backside of a wafer. In addition, since the area of the wafer increases as the square of unit increments of radius of the wafer, substantial portions of the area of a wafer are contained near the outer perimeter of the wafer. These areas of the wafer are oriented within the reactor closest to the cold walls of the chamber, and therefore tend to lose heat more rapidly via radiation to the adjacent cold walls than the center regions of the wafer which are more remote from the cold walls and which are essentially edge-wise "surrounded" by the integral perimeter portions of the wafer that are at substantially elevated temperature. Accordingly, larger wafers tend to present substantially greater difficulties in maintaining a desirable isothermal temperature distribution over the surface of the wafer for more uniform deposition over the entire surface of the wafer, and for better crystal quality in terms of dislocations commonly preferred to as `slip`.
The wafer temperature in conventional reactors is commonly controlled using banks of high-intensity lamps that are arranged outside the reactor chamber to irradiate the wafer through a cold-wall window in the reactor. However, as the decomposable gas flows through the reactor, some deposition occurs on the window that then becomes less optically transmissive and more absorptive of radiant flux from the lamps, with associated increase in window temperature, and increased rates of deposition on the window, in regenerative manner, with associated loss of thermal control of the wafer temperature as the window becomes less optically transmissive. For this reason, the reactor must be dismantled frequently in order to clean deposits from the window, with associated reduction in the throughput or production rate of wafers through the reactor over time.